Fate of E. coli O157:H7, Salmonella spp. and potential surrogate bacteria on apricot fruit, following exposure to UV-C light

Abstract

Some soft fruit, such as tree-ripened apricots, cannot be washed with aqueous sanitizers, due to their innate softness and delicate surfaces. In this study, ultraviolet-C (UV-C) light was investigated for its efficacy in inactivating 4–5 individual strains of Escherichia coli O157:H7 and Salmonella spp. on apricots, in addition to a four-strain composite of Shiga toxin-negative E. coli O157:H7 and a cocktail of three attenuated strains of Salmonella Typhimurium and Salmonella Typhimurium LT2. Also, the survival of E. coli O157:H7 and Salmonella spp. after exposure to 74 and 442mJ/cm2 of UV-C was evaluated during post-UV storage at 2 and 20°C. The fruit was spot inoculated and the areas (ca. 1.5cm2) of fruit surface with the inoculated bacteria were exposed to UV-C at 7.4mW/cm2. E. coli O157:H7 and Salmonella spp. populations decreased rapidly (1–2 logs) (P<0.05) with increasing UV-C doses of 0 to 74mJ/cm2. Further increases in UV-C dosage achieved only limited additional reductions in bacterial populations. Shiga toxin-negative bacteria and attenuated S. Typhimurium strains, along with S. Typhimurium LT2, responded similarly to corresponding pathogenic E. coli O157:H7 and Salmonella spp. During storage at 2 or 20°C, populations of pathogenic E. coli O157:H7 and Salmonella spp. on untreated fruit decreased slowly; however, populations on fruit treated with 442mJ/cm2 decreased rapidly at both temperatures. After 8days at 20°C or 21days at 2°C, E. coli O157:H7 and Salmonella spp. populations on UV-C treated fruit were at least 2logCFU/g lower than on non-treated controls. Our results suggest that surface-inoculated bacteria survived poorly following UV-C treatment of apricots.